Vacuum metallizing is a process that has a long history. U.S. patents issued to inventors who have gained great fame are present in pior art relating to this general field of technology--inventors such as Thomas A. Edison and Clarence Birdseye.
In general, this technology takes several forms. All forms require a vacuum chamber capable of producing reduced pressures in the range of 10.sup.-4 Torr (1 Torr=1/760 atmospheres pressure). The chamber may have fixed targets or substrates to be metallized in what might be called a product batch process, wherein the targets are removed after they are coated; or it may have a spooled web of finite length that is uncoiled as it passes over the metal vapor source to be wound on a takeup spool in what might be called a product semi-continuous process because the process must be stopped to change out the spools. The metal vapor source may be a batch device, wherein an intitial charge of metal is vaporized without being replenished (i.e., a source batch process), or it may be an evaporating source having a continuous supply of metal (i.e., a source continuous process), usually fed in the form of a continuous wire.
An ideal process would combine a source continuous process with a true product continuous process, wherein a continuous web of substrate material passes into the vacuum chamber, is exposed to a source continuous process therein, and exits the vacuum chamber as a coated web product. This technology is not now broadly available.
This invention is primarily directed to improving the overall production rate and efficiency of a source continuous, product semi-continuous, vapor depositing process while using existing commercial vapor plating equipment; in other words, retrofittable into the installed capacity of the industry. The disclosure is not to proscibe the use of this invention in other processes, but to point to the overall process wherein the teachings of this invention will be most efficacious.
A part of improving the overall production rate of such a process is the task of maintaining the cleanliness of the vacuum chamber, especially owing to the fact that spurious metal condensate within the chamber tends to have extremely high surface area that is capable of adsorbing inordinate amounts of atmospheric gases and vapors whenever the chamber is opened. These adsorbed gases and vapors add to the pumping load required to achieve the high vacuum required to operate the process, thereby they increase the pump-down time that is a part of the down time of the equipment, which increase adversely affects the overall production rate of the equipment. Thus, it can be seen that reducing spurious metal condensate within the vacuum chamber is a desirable end.
The equipment in the commercial vapor plating industry today is evaporator limited. That is to say, the rate of evaporation from the evaporators currently in use limits the production rate of the equipment. The limit is created by a phenomenon called "spitting", wherein particles of molten metal are emitted from the molten pool of metal and the particles strike the product substrate, thereby creating an unacceptable product. Spitting may be caused by evaporation of metal at the interface between the molten pool of metal and the containment boat, which is also the heating element. In aluminum vapor plating operations, spitting appears to begin occurring as the evaporation flux reaches approximately 0.059 g/cm.sup.2 /min (i.e., 3.8 g/in.sup.2 /min). Thus, although the present web handling equipment, the wire feed equipment, and the power supply equipment are all capable of allowing production rates up two to four times the present capacity of the existing equipment, the spitting phenomenon effectively limits the capacity of the process equipment. Most commercial equipment, therefore, operates at a maximum rate in the range of 0.031 g/cm.sup.2 /min to 0.059 g/cm.sup.2 /min (i.e., 2.0 to 3.8 g/in.sup.2 /min). Thus, it would be a clear improvement to increase the evaporation flux level that results in spitting or to otherwise reduce or eliminate the effects of spitting on the product quality. It is known that the onset of spitting is also affected by the operating pressure; higher pressures inhibit spitting. Thus, in practice, the operating pressure is a variable that is subject to compromise; lower pressures result in a cleaner vacuum chamber by increasing the efficiency of deposition on the target by lengthening the mean free path of the metal molecules. Shortened mean free path, resulting from higher vacuum chamber pressures leads to increased spurious condensation as well as raising issues of oxidation or other reactions with the residual gases in the chamber, leading to potentially reduced product quality.
The evaporator boats that are used in common commercial practice today are both containment vessel and heating element. Typically, an evaporator boat comprises a heating element with a cavity hollowed out of it to provide a well for the metal to be evaporated. The material of which the heating element is made must be an electrical conductor having suitable resistivity. Often intermetallic compounds of titanium diboride (TiB.sub.2) and boron nitride (BN) in binary mixture are used and occasionally in ternary mixture with aluminum nitride (AlN). Small quantities of tungsten or calcium compounds might also be included (as in U.S. Pat. No. 4,373,952, 1983, Parcut, and U.S. Pat. No. 4,526,840, 1985, Jarabek). The heating element must be resistant to the corrosive and erosive effects of the metal in both the molten and vapor states. Sometimes this chemical resistance is provided by means of a resistant coating (as in U.S. Pat. No. 4,264,803, 1981, Shinko).
From the foregoing paragraph, it can be seen that the evaporation source in common commercial use is an article with many functions, and therefore must require a comprise of some properties to achieve others. It is clear that if the functions of the existing evaporator could be provided by separate entities, the physical and chemical properties of each entity could be optimized to more effectively perform the specific function assigned to the entity. The resulting evaporator may be more complex, being assembled from multiple entities or pieces, but if it may be retrofitted into the existing commercial equipment and if it increases the productivity of that equipment, it will clearly advance the state of the art.